In some cases, a steel gear has problems in which engaging noise caused by transmission error or tooth beating sound caused by backlash results in vibrations and noise. Compared with this, a plastic gear in which a portion of the gear tooth is formed from plastic, has a merit of possessing physical properties of viscoelastic behavior of the plastic, that is, large attenuation and low rigidity, and therefore, it is possible to complete the gear with low vibrations and low noise. Further, since the plastic gear is lightweight, and has the advantage that the plastic gear is obtained at low cost since gear-cutting work is not necessary because the tooth portion can be formed just by injection molding; and thus replacement from the steel gear advances, so that the plastic gears are broadly used in OA equipment, or audio products. However, the above physical properties providing low vibrations and low noise become constraint, so that, in almost cases, the plastic gears are used in light load, low revolution and at ambient temperature.
As a next application of the plastic gear from the contemporary condition, if it is possible to realize application to a driven gear within engine such as, for instance, the balancer weight drive gear or the camshaft timing gear, of the four-wheel car or the two-wheel vehicle, it is extremely effective for noise reduction. However, suppose that the plastic gear be applied to the driven gear within engine, the plastic gear is naturally used under the conditions of high load, high revolution and high temperature, and it should be considered that impact load is added to the plastic gear or the plastic gear comes into contact with engine oil and the like, and further, since the plastic gear be used with combination of the steel gear, it is necessary for the plastic gear to have high fatigue strength, high abrasion resistance, high heat resistance and high impact resistance compared with conventional use, in addition, properties of high oil resistance, high gasoline resistance, and low aggression to the opposing steel gear are required.
As a method to improve mechanical strength of the plastic gear, for instance, JP8-156124A and JP8-174689A have proposed the plastic gear in which fabric cloth of cylindrical aramid fiber is arranged periphery of the metal insert, a plastic portion is formed in such a way as to impregnate thermosetting resin material of phenol resin or the like to the fabric cloth, and after fabricating disk shape plastic molding (gear blank), gear-cutting work is performed to the plastic portion to form gear shape. However, it is necessary for the plastic gear to be performed gear-cutting work of the plastic portion, after forming the gear blank, and thus there is the problem of becoming high cost caused by increase of number of man-hour. Further, there is also the problem that, as the property of the thermosetting resin material, the impact resistance is low, and cycle time at the time of manufacturing becomes long due to heat treatment.
On the other hand, performed is the attempt of forming the gear tooth by only injection molding without going through configuration of the gear blank, and fabricating the gear without performing the gear-cutting work. For instance, JP55-41273A has disclosed the plastic gear using the thermoplastic resin material, and further, JP2-8542A and JP2005-214338A have disclosed the plastic gear in which the plastic portion is made two-layer structure of the obverse layer tooth and the internal layer tooth, and the thermoplastic resin material of including minute quantity of reinforced fiber or not including reinforced fiber is used in the obverse layer tooth, while the thermoplastic resin material of highly including the reinforced fiber such as glass fiber is used in the internal layer tooth. Although these plastic gears intend improvement of the fatigue strength and the abrasion resistance, and reduction of the aggression to the opposing steel gear, sufficient improvement with respect to the impact resistance is not achieved.
To this problem, for instance, JP2000-145928A and JP2004-150518A have proposed the method for adding elastomer to the gear main body portion. However, the methods disclosed therein have the problem in which cost becomes high caused by increase of the number of parts and man-hour of fabrication. In addition, JP2001-208166A and JP2002-156025A have disclosed the joined structure of the plastic portion forming the gear tooth and the metal insert; however, both cases are structures in which entirety of the plastic portion is fixed to the metal insert, and thus improvement of the impact resistance is not considered.